Conventionally, autothermal reformers are stand alone reformers with no tubular reformers up-stream. Start-up of the autothermal reformers has been performed by preheating the entire reactor to high start-up temperatures i.e. between about 600.degree. C. and 1000.degree. C. After the start-up temperature has been reached, a feedstock of e.g. gas steam mixtures and oxygen steam mixtures or in some cases oxygen air-mixtures is introduced into the heated reactor for further processing. The reactor is thereby subjected to detrimental pressurization and refractory lining material in the top part of the reactor and to excessive heat-up, during the first period of operation until a constant high pressure has been established.
During start-up, autothermal reformers produce a product gas mixture containing soot particles leading to a dark condensate, which cannot or only after further processing be reused. Other methods of starting similar reactors have been used in the industry.
In the ammonia industry, where a tubular reformer supplies heat for preheating of a secondary reformer when added to the secondary reformer so that the secondary reformer is not subjected to abrupt pressure changes and only to a modest increase in temperature.